Selective killing of particular types of cells is desirable in a variety of clinical settings, including the treatment of cancer, which is usually manifested through growth and accumulation of malignant cells. An established treatment for cancer is chemotherapy, which kills tumor cells by inhibiting DNA synthesis or damaging DNA (Chabner and Roberts, Nat. Rev. Cancer 5:65 (2005)). However, such treatments often cause severe systemic toxicity due to nondiscriminatory killing of normal cells. Because many cancer chemotherapeutics exert their efficacy through selective destruction of proliferating cells, increased toxicities to normal tissues with high proliferation rates, such as bone marrow, gastrointestinal tract, and hair follicles, have usually prevented their use in optimal doses. Such treatments often fail, resulting in drug resistance, disease relapse, and/or metastasis. To reduce systemic toxicity, different strategies have been explored to selectively target a particular cell population. Antibodies and other ligands that recognize tumor-associated antigens have been coupled with small molecule drugs or protein toxins, generating conjugates and fusion proteins that are often referred to as immunoconjugates and immunotoxins, respectively (Allen, Nat. Rev. Cancer 2:750 (2002)).
In addition to dose-limiting toxicities, another limitation for chemotherapy is its ineffectiveness for treatment of cancers that do not involve accelerated proliferation, but rather prolonged survival of malignant cells due to defective apoptosis (Kitada et al., Oncogene 21:3459 (2002)). For example, B cell chronic lymphocytic leukemia (B-CLL) is a disease characterized by slowly accumulating apoptosis-resistant neoplastic B cells, for which currently there is no cure (Munk and Reed, Leuk. Lymphoma 45:2365 (2004)).
Cancer stem cells (CSCs) are a small fraction of tumor cells that have a capacity for self-renewal and unlimited growth, and therefore are distinct from their progeny in their capacity to initiate cancers (Schulenburg et al., Cancer 107:2512 (2006)). Current cancer therapies do not target these cancer stem cells specifically, and it is hypothesized that the persistence of CSCs results in an ineradicable subset of cells that can give rise to progeny cells exhibiting drug resistance and/or contributing to the formation of metastases. In those tumors which harbor CSCs it is highly desirable to be able to eliminate these cells. CSCs have been thought to possess many properties similar to that of normal stems cells, e.g., long life span, relative mitotic quiescence, and active DNA repair capacity, as well as resistance to apoptosis and to drug/toxins through high level expression of ATP-binding cassette drug transporters such as P-glycoprotein. Consequently, CSCs are thought to be difficult to target and destroy by conventional cancer therapies (Dean et al., Nat. Rev. Cancer 5:275 (2005)). Conversely, it is critically important to distinguish CSCs from normal stem cells because of the essential roles that normal stem cells play in the renewal of normal tissues.
To increase the selectivity of highly toxic anti-tumor agents, various attempts have been made to take advantage of specific features of the tumor microenvironment, such as the low pH, low oxygen tension, or increased density of tumor specific enzymes, that are not found in the vicinity of normal cells in well-perfused tissues. Environmentally sensitive anti-tumor agents have been developed that are hypothesized to exhibit increased toxicity in the solid tumor. For example “bioreductive prodrugs” are agents that can be activated to cytotoxic agents in the hypoxic environment of a solid tumor (Ahn and Brown, Front Biosci. 2007 May 1; 12:3483-501.) Similarly Kohchi et al. describe the synthesis of chemotherapeutic prodrugs that can be activated by membrane dipeptidases found in tumors (Bioorg Med Chem. Lett. 2007 Apr. 15; 17(8):2241-5.) The use of selective antibody conjugated enzymes to alter the tumor microenvironment has also been explored by many groups. In the strategy known as antibody-directed enzyme prodrug therapy (ADEPT), enzymes conjugated to tumor-specific antibodies are intended to be delivered to the patient, followed by a chemotherapeutic agent that is inactive until subject to the action of the conjugated enzyme (see for example Bagshawe, Expert Rev Anticancer Ther. 2006 October; 6(10):1421-31 or Rooseboome et al. Pharmacol Rev. 2004 March; 56(1):53-102) To date the clinical advantages of these strategies remain undocumented and there remains a high interest in developing more selective and more potent agents that can show therapeutic utility.